Move pxpath bin path to variable

Build checks are the first customer.

src/pxpath.sh.skel

 # libxml2 and its Python bindings.
 #
 
-
 pxpath() {
     #
     # Run XPath query $1 on file $2, register namespaces $3..
             }
     esac
     #shellcheck disable=SC2154
-    "__SHELLFU_LIBEXEC__"/pxpath \
+    "$_PXPATH__BIN" \
         "$query" "$file" "$@"; es=$?
     test -n "$tmp" && rm "$tmp"
     return $es
 # INTERNAL # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
 #          #                                not even as XPath expression #
 
+#
+# Path to pxpath python script
+#
+_PXPATH__BIN=${_PXPATH__BIN:-__SHELLFU_LIBEXEC__/pxpath}
+
 __pxpath__usage() {
     #
     # Print usage message for function pxpath__$1